BPVC Section VIII Division 2 defines primary membrane stress in a region of stress as local if the distance over which equivalent linearized stress exceeding 1.1 Sm does not exceed √(Rt) in the meridional direction. Current edition of BPVC Section VIII Division 2 code does not define meridional distance for primary membrane stress between Sm and 1.1 Sm. As a result, users are forced to treat the region between Sm and 1.1 Sm as general primary membrane stress which leads to a potential for failure in stress analysis or the need to use more costly elastic-plastic analysis.
Existing research and data by Ferry et al. on ASME Section III in 1983 indicate that there is a need to address meridional distance where stress is below 1.1 Sm . Currently there are no limits on meridional distance when localized primary membrane stress is between Sm and 1.1 Sm. This paper presents possible scenarios with lower primary membrane stress. Considering the same stress gradient (the change in stress to the change in meridional distance) permissible in current edition of the code, this paper suggests meridional distance limits 1.25√(Rt) when maximum local primary membrane stress is between Sm and 1.1 Sm.